How to Install Metal Roofing Over Plywood

Quick answer: Yes, you can install metal roofing directly over a plywood deck – but never onto bare plywood. The sheathing has to be at least 7/16″ thick (15/32″ or 1/2″ is the safe default), dry, and flat, and it needs a layer of underlayment between the wood and the panels. Most failures on this kind of roof don’t come from the metal. They come from skipping the underlayment or fastening into plywood that was too thin to hold a screw.

That’s the short version. Below is the full process, the materials, the thickness numbers, and the mistakes that cost people a re-do.

Can metal roofing go straight onto plywood?

Plywood (and OSB) is the most common roof deck in North American homes, so this comes up constantly. Metal panels – corrugated, ribbed, or standing seam – sit perfectly well on a plywood deck as long as two things are true:

1. The plywood is thick enough and structurally sound.
2. There’s a barrier between the metal and the wood.

What you should not do is lay panels right on the plywood. Metal and wood expand and contract at different rates, and the underside of a metal panel sweats when warm air meets a cold surface. Without underlayment, that moisture sits on the sheathing and rots it from the top down. People don’t notice for years, and then a whole section is soft.

So the rule is simple: good deck, then underlayment, then metal.

Plywood thickness for a metal roof

Thickness is where a lot of DIY jobs go wrong. Too thin and the screws strip out the first time the wind loads the panels. Here’s what works on a typical residential rafter spacing of 16″–24″ on center.

For a roof deck you want exterior-grade, weather-resistant sheathing – not interior board that delaminates the first time it gets damp during the build. A WBP (weather and boil proof) plywood or properwaterproof plywood is the right call here, because the deck will see moisture before the panels go on, and it’ll see condensation for the life of the roof. If you’re matching a specific spec, the 15mm and 18mm sheets are the two sizes most roofers reach for.

One more thing: check the panel manufacturer’s instructions. If they say “minimum 1/2″ plywood,” that number is tied to their warranty. Going thinner can void it.

What you’ll need

A note on fasteners: don’t cheap out. The rubber washer is what keeps water out of every single hole you drill, and there can be a thousand holes on one roof. Bad washers = a thousand small leaks waiting to happen.

Step-by-step installation

1. Inspect and prep the deck

Walk the whole deck before anything goes down. Any spongy spots, water stains, or popped nails – fix them now, because you won’t reach them later. Sheathing seams should land on a rafter, not float in mid-air. Renail loose panels and sweep it clean. A nail head sticking up will telegraph right through a metal panel and show as a dimple.

If you’re decking from scratch, leave a small gap (about 1/8″) between sheets so the plywood can expand without buckling.

2. Snap your reference lines

Measure and chalk a square line up from the eave. Metal roofing is unforgiving – if your first panel is off by half a degree, that error multiplies across the whole roof and the last panel won’t fit. Spend the extra ten minutes here.

3. Lay the underlayment

Start at the eave. Roll out synthetic underlayment horizontally, overlapping each course by about 4″–6″, and work your way up so water always sheds over the seam, never into it. At the eaves, in valleys, and around penetrations, use a self-adhering ice & water shield instead – that’s where leaks start.

This layer is the whole reason your plywood survives. Don’t skip it, and don’t substitute a cheap felt that’ll bake apart under a hot metal roof. Use a high-temperature-rated underlayment.

4. Install eave and gable trim

Fasten the eave drip edge and any starter trim first. These give your panels a clean edge to lock against and stop wind from getting under the leading edge.

5. Set the first panel

Line the first panel to your chalk line with a slight overhang past the eave (3/4″–1″ is typical) so water drips clear of the fascia. Get this one dead straight. Tack it, double-check square, then fasten.

6. Fasten correctly

Drive screws straight, not at an angle, and seat them so the rubber washer compresses just slightly – not crushed, not loose. On exposed-fastener panels, screw into the flat of the panel (or the rib, depending on the system – follow the panel’s guide), hitting solid plywood every time. Typical spacing is roughly every 12″–24″ along the supports, but the manufacturer’s pattern wins.

7. Overlap the next panels

Each panel overlaps the previous by one rib. Run a bead of butyl sealant in the overlap on low-slope roofs. Keep checking that you’re staying square as you go across.

8. Cut around penetrations

For vents, chimneys, and pipes, cut openings with snips and seal with a proper boot or flashing plus sealant. Cut metal makes hot, sharp swarf – brush it off the panels right away or it rusts and stains the finish.

9. Close it out

Install valley flashing, then the ridge cap last, with closure strips underneath to block wind-driven rain and pests. Walk the roof, snug any screw you under-drove, and clean off every metal shaving.

Underlayment: the part people regret skipping

If you take one thing from this guide: the underlayment between plywood and metal is not optional. Metal roofs breathe temperature swings all day. Warm attic air hits the cold underside of the panel, condenses, and drips onto the deck. Synthetic underlayment gives that moisture a surface to run off instead of soaking the plywood.

On low-slope sections (below about 3:12), go further – full ice & water shield across the field, not just the edges.

Common mistakes

Plywood vs. OSB for a metal roof deck

Both work. OSB is cheaper and very common. Plywood holds screws a bit better, handles repeated wetting and drying with less swelling, and stays flatter over time – which matters under metal, because every bump shows. If the roof will see real weather exposure during the build, or you’re in a wet climate, exterior WBP plywood is the more forgiving choice. For added fire performance in certain zones, there’s also fire-retardant plywood graded for roof decking.